Process for forming an iron-chromiumaluminum coating on a ferrous article



United States Patent PROCESS .FOR FORMING AN IRON-CHROMIUM- ALUMINUM COATING ON A' FERROUS ARTI- CLE Harold E. Bellis, North Tonawanda, N.Y., and Giles F. Carter, Wilmington, Del., assignors to 'E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Oct. 15, 1965, Ser. No. 496,709

4 Claims. (Cl. 117-114) This application is a continuation-in-part of application Serial No. 335,378, filed January 1, 164, which is a continuation-in-part of application Serial No. 211,895, filed July 23, 1962, which is in turn a continuation-inpart of application Serial No. 139,369, filed September 20, 1961, said applications'being now abandoned.

This invention related to a process for preparing articles which exhibit outstanding corrosion resistance properties comprising a ferrous metal substrate having a ferritic chromium-aluminum-iron alloy coating that may or may not also include small amounts of nickel as an alloying element, wherein the surface composition of said coating is controlled to contain at least 12% by weight chromium and a content of aluminum within specified critical limits. H

Metal coatings on dissimilar metal substratesare a known means of providing surface protection. Ferritic stainless steel coatings have been provided on ferrous metal articles in the past to provide corrosion resistance properties not possessed by the base metal origin-ally. Ferritic stainless coatings have been formed on ferrous metal articles, for example, by cladding a sheet of ferritic stainless steel to a sheet of base metal such as mild steel. It has also been proposed to form such coatings on ferrous metal articles through chromium 'difiusion treatment whereby the surface of the base metal, such as mild steel or carbon steel, is alloyed with chromium. These priorferritic alloy coatings have been essentially chromiumiron alloys.

Although such prior ferritic stainless coatings have been useful in enhancing the corrosion resistance properties of lessexpensive ferrous base metals, there is great demand for achieving ferritic stainless coatings which can impart increased corrosion resistance characteristics to inexpensive base metals. Furthermore, it has been found that articles having known ferritic stainless coatings are usually not formable; the ferritic coating in many instances actually separating from the formable base metal when the coated article is deformed to any appreciable extent. On the other hand, articles of the prior art, having a ferritic alloy coating, which are formableare found to exhibit relatively poor corrosion resistance at'the plastically deformed areas. Apparently this condition is due to the fact that small cracks or fractures occur in the coating during forming so that the corrosionsusceptible base metal becomes exposed to corrosive media and rust appears on the surface of the coated articles.

It is the object of the present invention to. provide an article of manufacture exhibiting increased corrosion resistance comprising a ferrous metal substrate having a ferritic chromium-aluminum alloy coating which may or may not also include nickel as an alloying element,-said coating containing at its surface at least 12% by weight 7 ing process.

3,287,159 Patented Nov. 22, 1966 'ice chromium and an appreciable content of aluminum but .which is maintained below an upper critical limit.

rous metal article having an iron-chromium alloy coating is markedly increased on the addition of "aluminum to the coating so that top 0.3 mil of said resulting coating is ferritic and consists essentially of iron, at least 12% by weight of chromium and from about 0.08% to 4% by Weight of aluminum. It has been found that if the content of aluminum in the top 0.3 mil of the coating reaches about 5% by weight the corrosion resistance of the coated article actually decreases over a ferrous metal article possessing a ferritic chromium-iron alloy coating. It has been found that nickel may be alloyed with the chromium-aluminum-iron alloy coating defined above, if desired, without adversely affecting the improved corrosion resistance performance obtained if the nickel content in the top 0.3 mil of the coating is maintained below approximately 6% by weight. The presence of nickel infthe coating is found to offer animportant advantage inasmuch as it confers a self-healing characteristic to the ferritic alloy coating. If a coating containing nickel asan alloying element develops a rust spot, the rust may be removed from the spot, and although a mark or scar remains at the site, the coating resists the continued showing of rust. By contrast, when the coating does not contain nickel, if rust is removed, more rust continues to form at the spot from the ferrous metal underlying the coating and, of course, this rust detracts from the visual attractiveness of the coated article.

Articles according to the present invention may be prepared by diifusion processes, as Well as by any other coating processes in which the coating is metallurgically bondedto the base metal, such as, cladding by rolling,

cladding by high energy impaction, soldering, fusion joints, casting one metal to another or by any other coat- The base metal underlying the coating may be any ferrous metal, such as, for example, cast iron, carbon steel, mild steel, low-carbon steel, stainless steel, low-alloy steels, such as those containing small amounts of chromium, niobium, vanadium, titanium, zirconium, or

the like. The outstanding economic advantage offered by the coated articles of the invention, of course, is that a relatively inexpensive base metal, such as mild steel, may be provided the corrosion resistance characteristics of a very superior ferritic stainless steel with the use of only a very thin surface coating. The articles of the invention, for example, are found to possesssuperior corrosion resistance over many 300 and 400 series solid stainless steel products.

Although any desirable coating technique may be emplayed, the preferred means for obtaining the coated articles of the invention, is the diifusioin process disclosed in US. Patent No. 3,184,331, which is assigned to our assignee. The process vinvolves the diffusion of various elements including chromium and nickel into the surface of a ferrous metal article by use of a metal transfer agent selected from the group consisting ofcaloium, banium, magnesium, and strontium, whereby the transfer agent acts principally as a solvent and transfer medium to bring the diffusing elements into contact with the of the diffusing elements, dictated by well-known laws of solid state diffusion, then causes coating growth. The preferred embodiment for executing the dilf usion process is by immersing the base ferrous metal article in a molten bath of the metal transfer agent having incorporated therein a. source of .the desired diffusing elements wherein said contacting is carried outaat a temperature between about 800 C. and the melting point of said article. Calcium is the preferred transfer agent and the process when using calcium is preferably carried out at a temperature between about 1000 to 1200 C. A more complete description of the process and its theory of operation is disclosed in the aforementioned United States patent, which is incorporated herein by reference.

It has been found in accordance with the present invention that the iferritie :alloy composition within the range required for the coated surface of thearticles of the invention can be obtained by maintaining the amounts of chromium, aluminum, and nickel in the metal transfer bath based on the amount of transfer agent present therein during the diffusion process. Within the range permissible for aluminum in the coating; namely,-about 0.08% to 4% by weight, the concentration of aluminum in the top 0.3 mil of the fern't-ic alloy coating formed on the ferrous base metal is approximately equivalent to 1.5 to 2 times the concentration of dissolved aluminum in the bath based on the amount of transfer agent present. The amount of aluminum in the bath for the diffusion process, therefore, may range from about 0.04% to about 3% by weight based on the amount of transfer agent present in the bath. Within the range permissible for nickel in the coating; namely, up to approximately 6% by weight, the concentration of nickel in the top 0.3 mil of the ferritic alloy coating formed on the ferrous base metal is approximately equivalent to the amount of nickel present in the bath. The amount of nickel in the bath for the diffusion process, therefore, may range up to approximately 6% by weight based on the amount of transfer agent present. Chromium is completely miscible in iron and only slightly soluble in the transfer agent, the amount of chromium soluble in calcium, for example, at 1000 C., being in the order of about 0.1% by Weight. Due to this fact, a saturated solution of chromium in the transfer agent favors rapid diffusion of chromium into ferrous base metals.

For usual coating times, of from to 60 minutes, it is preferred that in executing the d-ilfusion process that the amount of chromium in the bath, preferably in finely divided form, be in a range of from 3% up to 10% by weight based on the amount of transfer agent present therein. This provides an excess of chromium insuring a saturated solution of chromium in the transfer agent for optimum liquid-to-solid transfer of chromium to occur. Amounts of chromium above 10% by weight based on transfer agent may be used but little or no advantage is gained thereby. It will be appreciated from the foregoing that amounts considerably less than 3% by weight may be used successfully but usually it is not commercially feasible to operate with the amount of chromium in the bath below approximately 0.25% by weight based on the amount of transfer agent present.

The coatings of the articlesof the invention formed by the above-described diffusion process may contain small amounts of diffused transfer agent. The coatings of the novel articles of the invention may also contain small amounts of other elements in addition to chromium-aluminum-iron which may diffuse into the coating from the ferrous metal substrate during the coating process or which may be purposely added so long .as the novel and basic characteristics with respect to improved corrosion resistance set forth herein for the novel ferritic alloy coating of chromium-aluminum-iron are not altered.

A better understanding of the invention will be gained from the following examples demonstrating the features and advantages of the articlesof the invention. These articles illustrating the invention were prepared by means. of the above-described diffusion process and a showing of typical preparations by such means are included. It

is to be appreciated that the ferritic alloy coatingsof these articles formed by a diffusion coating technique are characterized by greater concentrations of the diffusing elements chromium, aluminum, and nickel at the outer surface of the coating and decreasing concentrations of these metals downward to the ferrous metal substrate.

the surface of the ferrit-ic alloy coating which are established by the following examples as limits below which aluminum and nickel must be maintained in orderto gain the improvements of the invention constitute as well the maximum content of such alloying elements that may be tolerated at any level in the ferritic alloy coating for the articles of the invention.

In the following examples, amounts of the various ingredients are given in terms of percent by Weight, unless otherwise indicated. All samples prepared were cooled to room temperature by quenching in oil upon removal from the coating bath. The concentrations of chromium, aluminum, and nickel reported represent. a

measure of their average concentrations in approximately the top 0.3 mil of the coating as determined by X-ray fluorescence. The surface of each sample was lightly polished before the concentration was determined. The thicknesses of the coatings reported were determined by microscopic examinations of cross-sections of the coated articles, after etching by 3% by weight concentrated nitric acid, 97% by weight ethanol in 30-60 seconds.

Example 1 A series of specimen articles were prepared comprising a ferrous metal substrate having a ferritic alloy coating with the top 0.3 mil of said coating having a.

grams calcium, grams of powdered chromium (-325 mesh), and 12 grams of aluminum was heated to 1100 C. The bathwas agitated by a mechanical stirrer and was covered by an inert atmosphere of argon. A mild steel specimen containing 0.04% carbon was introduced for a treating time of 28 minutes. The coated specimen was then removed from the bath and was cooled to room temperature. A coating of 1.7 mils was formed on the base metal, the top 0.3 mil of said coating having a concentration of 28% chromium and 1.1% aluminum; The coating procedure was repeated to prepare-specimens listed in Table I with the amount of aluminum in the coating bath and coating time being varied Ias indicated in the table.

A series of such specimens each measuring 2" x 4" were then tested for corrosion resistance on the flat coated surface of the specimen (without deformation) in accordthis test, the sample is subjected to an acetic acid salt spray solution to which small amounts of copper chloride are added to promote corrosion. The test is now in broad use throughout the portion of the chromium plating in- Therefore, it can be clearly expected that the 1 maximum limits for aluminum and nickel contents at.

'dustryconcerned with out-of-door durability, being regarded as an excellent accelerated corrosive test which similates the corrosion behavior and durability of chromium plated steel and zinc alloy parts in out-of-door service. The specimens were inspected for rust spots on the 2" x 4" coated surface after 16 hours and every 24 hours thereafter through 112 hours in the test with the total number of rust spots observed after each inspection being reported as the result of the test.

. Data tabulated in Table I below'are representative of the increased corrosion resistance provided by articles of the invention comprising a ferritic chromium-aluminumiron alloy coating formed on a mild steel substrate over the corrosion resistance provided by a coated article comprisinga ferritic chromium-iron alloy coating formed on the same mild steel substrate and that provided by coated articles having a ferritic chromium-aluminum-iron alloy coating but wherein the aluminum content significantly exceeds 4% in the top 0.3 mil of said coating. Each run reported represents six (6) 2" x 4" panels having the same coating composition so that the rust spots reported for each exposure period to the CASS Test is the total number observed for 48 sq. in.

ure for the test was considered to be ten visually observed rust spots on the coated surface of the specimen. The specimens were observed after 16 hours and every 24- hours thereafter until failure occurred; with the hours to failure being reported as the result of the test.

TABLE II Percent Percent Percent Coating CASS Lite N Al Cr Thickness In Hours To Failure The above results indicate clearly that nickel may be added as an additional alloying element in the ferritic coating up to amounts of about 6% without adversely affecting th improved corrosion resistance of the coated article of the invention.

TABLE I Composition of Coating CASS Hours of Al in Bath Coating Time Coating. percent Thickness Exposure Run No. (grams) (minutes) (mils) Al Cr 16 40 64 88 112 0 55 0 36 1. 82 4 17 18 20 C) 30 0.08 28 1. 12 0 6 6 S 8 3. 0 30 0. 30 33 1. 36 2 3 3 3 6 5. 8 28 0. 50 31 1. 32 0 2 2 1 1 9.6 28 0. 70 29 1. 41 l 4 2 5 11. 5 28 1. l 28 1. 66 4 4 3 6 6 38. 0 28 1. 8 31 3. 21 1 3 4 4 4 65. 0 50 2. 2 30 3. 39 1 5 5 5 7 92.0 56 3. 4 32 3. 81 3 3 5 7 6 127. 0 62 4. 9 33 4. 43 6 29 30 154. 0 10 6.8 28 2.5 72 118 Run carried out in container previously containing Ca-Al bath. No additional aluminum added".

It is obvious from the above results that only very small amounts of aluminum are necessary to effect significant increases in the corrosion resistance of a coated article having a chromium-iron alloy coating; 0.08% aluminum offering a surprising improvement. As will be noted when the aluminum content is 4.9%, the article actually exhibits somewhat less corrosion resistance than an article having a chromium-iron alloy coating without aluminum being present. The maximum limit for aluminum in the ferritic alloy coating is therefore somewhere between 3.4 and 4.9% and may be reasonably set at about 4% Amounts of aluminum above 5% in the coating exhibit quite an adverse efiect on corrosion resistance. Example 2 To determine the effect of adding nickel to chromiumaluminum-iron alloy coatings, a series of specimen articles were prepared comprising a ferrous metal substrate having a ferritic alloy coating with the top 0.3 mil of said coating having a composition consisting essentially of iron, chromium, aluminum, and nickel in which the chromium content varied over a wide range above a minimum value of 12%, the aluminum content varied from 0.05 to about 4% and the nickel content was permitted to vary over a range above and below 6%, the articles in which the nickel concentrations in the top 0.3 mil of the coating were below 6% being within the invention and the articles in which the nickel concentrations in the top 0.3 mil of the coating were above 6% being outside the invention. Data tabulated in Table H below are representative of such series showing the effect on corrosion resistance caused by the addiiton of varying amounts of nickel. In this series of tests, for each run a specimen measuring 4" x 6" was exposed to the CASS Test. Fail- As will be obvious from the foregoing description, the thickness of the ferritic alloy coating for the novel articles of the invention may vary over a wide limit. It is preferred, however, that the ferritic alloy coating on the article, as thickness was determined for the articles of the examples, be at least about 0.5 mil. Usually there is little advantage in the coating thickness being in excess of from 3 to 5 mils but thicker coatings may be formed, if desired. Although, as it has been previously stated, the chromium content in the top 0.3 mil of the coating may range over a broad range above the 12% by Weight considered a minimum content necessary herein to impart stainless steel characteristics to the coating, it is most preferred that the content of chromium be in a range of from about 25 to 50% by weight.

The novel articles of the invention formed by the abovedescribed diffusion process are also found to exhibit the desirable property of having a lower tendency for other materials to adhere to the surface of the ferritic alloy coating. For example, when formed into cooking utensils, such as frying pans, it is found that eggs and meat may be fried on the coated surface with little or no fats or oils being present without these substances exhibiting a tendency to stick to the coated surface. Similarly, it has been found that it is extremely difficult to electroplate metallic layers over the surface of the coatings of these articles due to the failure of the electroplate layer to adhere to the coated surface of the article.

It is to be appreciated that although outstanding corrosion resistance has been shown for the novel articles of the invention, even increased corrosion resistance can be obtained by well-known post-treatment techniques for such purposes. For example, corrosion resistance can be markedly improved by passivating the article after coating in 7 50% nitric acid or 20% nitric acid-2% sodium dichromate solutions.

In like manner, many well-known treatments can be employed to improve the surface appearance of the coated article, if desired. For example; an improved surface finish can be obtained by cold-working the base metal to a mirror-finish before coating or, alternatively, the surface of the coated article may be cold-worked to improve surface appearance. The coated articles can also usually be polished or butfed after being formed without such treatment being deleterious to corrosion resistance.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not to be limited to the specific embodiments thereof except as defined in the appended claims.

We claim:

1. A process for forming a ferritic alloy coating on a ferrous metal article, the top 0.3 mil of said coating having a composition consisting essentially of iron, at least 12% by weight chromium, and from about 0.08% to 4% by therein at least 0.25% by weight chromium based on the l amount of calciurnin said bath and a concentration of dissolved aluminum from about 0.04% to 3% by weight based on the amount of calcium in said bath.

2. The process of claim 1 in which the contacting is carried out at a temperature between 1000 to 1200 C.

3. The process of claim 2 wherein said bath contains up to 6% by weight of nickel based on the weight of calcium in said bath.

4. The process of claim 1 in which the resulting coated ferrous metal article is quenched from above a minimum temperature of approximately 900 C.

No references cited.

ALFRED L. LEAVITI, Primary Examiner.

R. S. KENDALL, Assistant Examiner. 

1. A PROCESS FOR FORMING A FERRITIC ALLOY COATING ON A FERROUS METAL ARTICLES, THE TOP 0.3 MIL OF SAID COATING HAVING A COMPOSITION CONSISTING ESSENTIALLY OF IRON, AT LEAST 12% BY WEIGHT CHROMIUM, AND FROM ABOUT 0.08% TO 4% BY WEIGHT OF ALUMINUM, COMPRISING IMMERSING SAID ARTICLE IN A MOLTEN BATH MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 800*C. AND THE MELTING POINT OF SAID ARTICLE CONTAINING CALCIUM AS A METAL TRANSFER AGENT AND HAVING INCORPORATED THEREIN AT LEAST 0.25% BY WEIGHT CHROMIUM BASED ON THE AMOUNT OF CALCIUM IN SAID BATH AND A CONCENTRATION OF DISSOLVED ALUMINUM FROM ABOUT 0.04% TO 3% BY WEIGHT BASED ON THE AMOUNT OF CALCIUM IN SAID BATH. 